A disk drive is a device for recording data on the surface of a disk and reading it. A head having a signal transducer is positioned at a desired location on a track on which data is recorded, and it reads or writes data. As a means to recover an error which occurs when data is read or written by the head, there is an error recovery procedure (ERP) comprising various error recovery steps.
On a disk, a read error occurs as a result of various causes such as a flaw, ununiformity of the magnetic substance, or change of the magnetic substance with time. Generally, for errors occurring in the data portion of the disk, an ECC, a conventional error recovery code, is used to execute an error recovery process. Further, various recovery procedures are executed, such as change of the read gain, change of the offtrack, and change of the bias value of an MR (magnetoresistive) element if it is used as the read head. If data is successfully reread by executing such recovery procedures, the data is continuously used. If the error cannot be recovered by a plurality of error recovery procedures, it is recognized as a hard error, or if the data can be reassigned to another area, the corresponding area is made an unusable area and a process for reassigning the data to the other area is performed.
In many of the recent disk drives, a magnetoresistive (MR) transducer head is used. This utilizes a very small resistance change due to change in a magnetic field By causing a d.c. current to flow through the MR element, a resistance change is converted to a voltage signal to perform the reading of data.
However, one of the read errors, which occurs in this approach of reading a resistance change, is thermal asperity. A thermal asperity means that a protrusion or deposit on a disk surface crashes against the read head and generates a resistance change due to a temperature change in the MR element, causing an abnormal signal.
One of the countermeasures against a thermal asperity is a technique called low rpm burnish. A low rpm burnish is an approach in which the rpm (revolutions per minute) of the disk is decreased to reduce the fly height of the magnetic head, thereby to positively scrape off the protrusion on the disk which caused the thermal asperity. This may also form one of the abovementioned error recovery procedures (ERPs).
There are various countermeasures against errors during data reading or writing. These are generally stored as a series of steps in an error recovery procedure. When the error recovery procedure is started, these steps are sequentially executed.
The causes of errors are not always the same, and there may be various causes. Accordingly, error recovery procedures meeting these various causes of errors are required. An error recovery procedure generally performs a rereading by changing and adjusting the standard read conditions one by one, which are defined among the disk, magnetic head, and HDC (hard disk controller). The read conditions may include, for instance, offtrack amount which is an offset amount between the center of the magnetic head and the center of a track, bias current value given to an MR element if the magnetic head includes the MR element, adjustment of the auto-gain control (AGC) for making the amplitude of a reproduced signal constant, adjustment of the speed and gain of the PLL circuit for stabilizing the sampling frequency (making the tracking speed a predetermined speed), and the like.
Normally, a plurality of error recovery steps are registered as an ERP. Those steps are executed in a predetermined order. A reread is performed for each step, and the ERP terminates when the reread has succeeded. If the reread is not successful, the ERP terminates when a preset maximum number of rereads is reached or the last step of the ERP is ended. Additionally, the ERP may be terminated by the time-out of a read or write instruction.
For a time-out, the host system may perform a retry of the instruction in the time-out state after executing a reset (soft reset or hard reset) for the disk drive. If a write or read error occurs again during the retry, the ERP is executed from the first step.
It may take 10 to 20 seconds to execute the steps of the above low rpm burnish. On the other hand, the time elapsing before the time-out of a read or write instruction depends on the operating system (OS) or BIOS of the host system, and conventionally, it was often about 30 seconds, but it may be only several seconds in the recent OSs such as OS/2 (a trademark) of IBM Corp. For this, a time-out of the read or write instruction may be reached before all ERP steps are completed In such a case, only some of the ERP steps are executed because of the limited time to the time-out, and thus there is a high possibility that, even if there is an effective step after that, an error occurs without the execution of that step. Further, for a read or write instruction to a plurality of sectors, if errors occur in a plurality of sectors, the ERP steps are executed in each sector, and thus the read or write instruction may reach the time-out before the completion of the ERP steps for the all sectors in error even if the time to the time-out is relatively long.